For centuries it has been known that glassy materials can be formed under very low forming pressures, including pressures achievable solely with the human lung when heated above its softening temperature. (See, e.g., D. R. Uhlmann & N. J. Kreidl, Glass: Science and Technology, Academic Press, New York, 1990, the disclosure of which is incorporated herein by reference.) Decades ago this observation was expanded, and it was recognized that synthetic plastics could be processed in a similar manner. (See, e.g., E. A. Muccio, Plastic Part Technology, ASM International, Materials Park, Ohio, 1991, the disclosure of which is incorporated herein by reference.) Blow molding became the terminology used to collectively describe a number of different techniques for plastic processing that allow the net-shaping of complex geometries consisting of thin sections with a vast aspect ratio.
In a separate development, superplastically formable (SPF) metallic alloys were discovered, which exhibited plastic deformations far beyond the plasticities normally associated with metals, which were usually expected to be less than 10-25%. Indeed, when stable two-phase microstructures with grain sizes of less than 10 μm were processed in an environment, where the temperature was around 0.5 Tm, and at the same time subjected to gas pressures of up to 5 MPa in a controlled manner, outstanding plasticities of ˜500% were observed. (See, e.g., C. E. Pearson, Journal of the Institute of Metals 54 (1934) 111-124; and W. A. Backofen, I. R. Turner, D. H. Avery, JOM—Journal of Metals 16 (1964) 763, the disclosures of which are incorporated herein by reference.)
Despite the improved properties shown by these SPF alloys, the flow stresses involved in shaping them are still significantly higher than those in plastic or glass at their respective processing temperatures. Recently a new class of materials referred to as bulk metallic glasses (BMGs) have been developed that show a number of attractive properties, including very high strength, elasticity, and corrosion resistance. (See, e.g., W. L. Johnson, Mrs Bulletin 24 (1999) 42-56; T. C. Hufnagel, Scripta Materialia 54 (2006) 317-319; and M. F. Ashby, A. L. Greer, Scripta Materialia 54 (2006) 321-326, the disclosures of which are incorporated herein by reference.) In addition, these materials can be cooled at cooling rates of about 500 K/sec or less from their molten state to form objects of 1.0 mm or more thickness with substantially amorphous atomic structure. (See, e.g., U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, the disclosures of which are each incorporated by reference herein.) That these BMG alloys may be formed into articles that are substantially thicker than conventional amorphous alloys, which have typical processes thicknesses of ˜0.020 mm and which require cooling rates of 105 K/sec or more, gives rise to a wide-variety of potential bulk applications. However, with a few exceptions BMGs have shown no or very limited plasticity. (See, e.g., J. Schroers, W. L. Johnson, Physical Review Letters 93 (2004) 255506; and Y. H. Liu, et al., Science 315 (2007) 1385-1388, the disclosures of which are incorporated herein by reference.) This in turn has limited the applications to which BMGs may be applied. (See, M. F. Ashby & A. L. Greer, Scripta Materialia 54 (2006) 321-326, the disclosure of which is incorporated herein by reference.)
Despite this significant limitation, it has been recognized that in small dimensions BMGs can show significant plasticity. For example, Conner et al. have shown that the plasticity of BMG beams in bending increases significantly when the beam thickness is decreased below 1 mm. (See, R. D. Conner, et al., Journal of Applied Physics 94 (2003) 904-911, the disclosure of which is incorporated herein by reference.) Also, it was observed that for the majority of BMGs the plastic zone shielding a crack tip is less than 1 mm. (See, M. F. Ashby & A. L. Greer, Scripta Materialia 54 (2006) 321-326, the disclosure of which is incorporated herein by reference.) These results suggest that an ideal geometry for BMG applications should be limited in at least one dimension to below 1 mm for BMGs to express their full potential properties. As a result, to date the geometries achievable with the vast majority of BMGs have been quite limited.
Currently, two fundamentally different processing routes are used to shape BMGs. (J. Schroers, JOM—Journal of Metals 57 (2005) 35-39, the disclosure of which is incorporated herein by reference.) The first is direct casting or molding, where the BMG is simultaneously fast cooled to avoid crystallization during solidification and filled or pressed into the entire mold cavity. The coupling of the forming and cooling steps in these techniques makes the production of thin sections with high aspect ratio particularly challenging. Indeed, only a careful balance of process parameters makes this process at all commercially useful, and even then it is only usable for a very limited number of geometries. (See, J. Schroers & N. Paton, Advanced Materials & Processes 164 (2006) 61-63, the disclosure of which is incorporated herein by reference.)
The second processing technique, broadly referred to as plastic forming, takes advantage of the sluggish crystallization kinetics found in BMGs to decouple the forming and cooling steps. Specifically, the unique kinetics of BMGs result in a supercooled liquid region. In this temperature region the BMG first relaxes during heating from room temperature at the glass transition into a supercooled liquid before it eventually crystallizes at the crystallization temperature, the upper bound of the supercooled liquid region. (See, Busch, R., Jom—Journal of the Minerals Metals & Materials Society, 2000. 52(7): p. 39-42, the disclosure of which is incorporated herein by reference.) For some BMGs the, temperatures and flow stress for plastic forming are comparable to plastics. (See, e.g., J. Schroers, & N. Paton, Advanced Materials & Processes 164 (2006) 61-63; J. Schroers & W. L. Johnson, Applied Physics Letters 84 (2004) 3666-3668; J. Schroers, et al., Applied Physics Letters 87 (2005) 61912; and B. Zhang, et al., Physical Review Letters 94 (2005), the disclosures of which are incorporated herein by reference.) Within this temperature window some BMGs can exist as viscous liquids with viscosities below 106 Pa s at time scales of several minutes. (See, Waniuk, T., et al., Physical Review B, 2003. 67(18): p. 184203, the disclosure of which is incorporated herein by reference.) This processing window provides unique processing opportunities, including using techniques typically reserved for plastics. (See, J. Schroers, JOM—Journal of Metals 57 (2005) 35-39, the disclosure of which is incorporated herein by reference.)
The ability to plastically form BMGs in their supercooled liquid region was recognized in the early days of metallic glass research and various terminologies are used, including superplastic forming, thermoplastic forming and hot-forming. (See, e.g., See, e.g., H. J. Leamy, et al., Metallurgical Transactions 3 (1972) 699; C. A. Pampillo & H. S. Chen, Materials Science and Engineering 13 (1974) 181-188; Patterson and Jones, Materials research Bulletins, 13 (1978) 583, the disclosures of which are incorporated herein by reference.) This processing opportunity has been used for a wide range of applications, including net-shape processing, micro- and nanoreplication, extrusion, synthesis of amorphous metallic foams, superplastic forming of sheet material, and synthesis of BMG composites. (See, e.g., N. Nishiyama & A. Inoue, Materials Transactions Jim 40 (1999) 64-71; Y. Saotome, et al., Scripta Materialia 44 (2001) 1541-1545; Y. Saotome, et al., Journal of Materials Processing Technology 113 (2001) 64-69; J. Schroers, et al., J. Mems 16 (2007) 240; Y. Kawamura, et al., Applied Physics Letters 67 (1995) 2008-2010; D. J. Sordelet, et al., Journal of Materials Research 17 (2002) 186-198; I. Karaman, et al., Metallurgical and Materials Transactions A—Physical Metallurgy and Materials Science 35A (2004) 247-256; J. Schroers, et al., Journal of Applied Physics 96 (2004) 7723-7730; T. Zhang, et al., Science Reports of the Research Institutes Tohoku University Series A—Physics Chemistry and Metallurgy 36 (1992) 261-271; W. J. Kim, et al., Materials Science and Engineering A—Structural Materials Properties Microstructure and Processing 428 (2006) 205-210; H. Soejima, et al., Journal of Metastable and Nanocrystalline Materials 24 (2005) 531; J. Schroers, et al., Scripta Materialia 56 (2007) 177-180; and A. A. Kundig, et al., Scripta Materialia 56 (2007) 289-292, the disclosure of which are incorporated herein by reference.)
However, even though during plastic forming of BMGs fast cooling and forming are decoupled, thin section articles with a high aspect ratio are challenging to create when using techniques where the BMG is in physical contact with the mold. This is due to stick conditions between the BMG and the mold under plane-strain conditions, which retards radial movement (parallel to the mold) of the BMG. For example, when considering the thermoplastic forming technique shown in FIG. 1a, an attempt to expand a flat disc of BMG between the platens in the supercooled liquid regime is limited. The reason for this stems from the fact that friction between the mold and the BMG at the contact surface opposes the deformation and an incommensurably higher forming pressure is required compare to the early stages where the required forming pressure solely has to overcome the flow stress (See FIG. 1b). For example, when deforming a cylinder of BMG forming alloy Zr44Ti11Cu10Ni10Be25 having a diameter of 11 mm into a flattened disc of 60 mm diameter at 440° C., a pressure of 30 MPa is necessary, however the flow stress absent frictional forces is only 0.3 MPa.
This effect can be reduced to some degree by using lubricants, which results in some slippage. However, the improvement is quite limited and the use of lubricants sacrifices the otherwise excellent achievable surface finish.
In short, BMGs, when properly formed from the molten state at sufficiently fast cooling rates, have high elastic limits, typically in the range of from 1.8% to 2.2%. Further, these amorphous alloys may show substantial bending ductility of up to 100%, such as in the case of thin melt spun ribbons. In addition, amorphous alloys being capable of showing glass transition are further capable of forming a super-cooled liquid above the glass transition range and can be significantly deformed using very small applied pressure (normally, 20 MPa or less). However, despite these desirable physical properties and the large inherent formability of some BMGs, under currently available shaping techniques shapes which are requiring high strains are simply not accessible.
In essence, the prior art methods of shaping articles of BMG do not allow for the utilization of the full range of formability characteristics because these methods each require that the BMG make contact with the shaping apparatus during the majority of time required for the forming operation. Accordingly, a new and improved method for forming articles of BMGs, which allows for the full access to the processing characteristics of these materials, is needed.